Silicide-coated composites and method of making them



May 14, 1968 P. E. BLACKBURN ETAL Filed March 29, 1965 MOSi SiO SiO M0812 Mo WITH 5; Mo M si BELOW SATURATION MoSi Mo Si T Mo Si GRADIENT F|g.1 Mo3si Flg. 3

' Mo Si MOSi sao Flg. 2

sso Mo Si Mo Si Mo SATURATED Mo SATURATED WITH Si WITH Si MOSSI Mo si sio Flg. 4 Flg. 5

INVENTORS Paul E. Blackburn Joan B. Morruck United States Patent 3,383,235 SlLICIDE-COATED COMPOSKTES AND METHOD OF MAKING THEM Paul E. Blackburn, Lexington, and Joan B. Mattuck, Boston, Mass, assignors to Arthur D. Little, Inc., Cambridge, Mass., a corporation of Massachusetts Filed Mar. 29, 1965, Ser. No. 443,432 11 Claims. (Cl. 117-70) This invention relates to refractory metals and alloys and more particularly to refractory bodies formed primarily of molybdenum or tungsten.

There is a great demand for refractory metals and alloys which are capable of resisting the action of h at, particularly in oxidizing atmospheres. As one example we may cite the use of such metals and alloys in the manufacture of gas turbine blades which are continuously exposed to high-temperature combustion gases. As another example we may cite the use of such refractory metals and alloys in making permanent or reuseable molds for casting ferrous metals. In a copending application filed in the names of Richard S. Davis, Joan -B. Mattuck and John L. Engelke, Serial No. 443,435 filed March 29, 1965, now US. Patent 3,286,312, and assigned to the same assignee as this application, there is disclosed a unique type of mold for casting iron and ferrous alloys which may be reused a large number of times. The refractory metal and alloy systems disclosed herein are particularly well-adapted for forming such molds.

Among the so-called refractory metals those formed of molybdenum or tungsten or the alloys containing one or both of these are known to possess good structural strength and to have relatively high thermal conductivities at high temperatures, and in general to possess physical properties which make them particularly well suited for a number of high temperature uses. However, it is necessary to protect these metals and alloys from surface oxidation, which of course is much accelerated at high temperatures. It has been customary to prevent this surface oxidation and subsequent deterioration of molybdenum and tungsten and their alloys by coating the surface with a protective coating material such as a silicide. For example, molybdenum disilicide is used as a protective coating on molybdenum and containing alloys; and tungsten disilicide has been used to protect tungsten and alloys containing it. However, such protective coatings have afforded only transitory protection inasmuch as the disilicide coatings are lost by virtue of the diffusion of the silicon into the metal or alloy onto which they are coated. Thus the effectiveness of the coating is lost after a relatively short exposure of the coated Substrate body to elevated temperatures. It would therefore be desirable to have available a refractory metal or alloy system to which a protective coating could be applied, the protective coating being one which was not subject to diffusion and hence deterioration.

It is therefore a primary object of this invention to provide a refractory metal or alloy system with a coating capable of protecting the system from oxidizing atmospheres at elevated temperatures for an extended period of time. It is another object of this invention to provide refractory metals and alloys containing large percentages of molybdenum and tungsten which have a coating ca pable of protecting them at high temperatures. It is yet another object of this invention to provide such a metal or alloy system which retains the physical properties of the metal or alloy while at the same time being adequately protected from high temperature oxidizing conditions. Other objects of the invention will in part be obvious and will in part be apparent hereinafter.

The invention accordingly comprises the several steps and the relation of one or more of such steps with re- 3,383,235 Patented May 14, 1968 ICC spect to each of the others, and the article possessing the features, properties and the relation of elements which are exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention reference should be had to the following detailed description taken in connection with the accompanying drawings in which FIGS. 1-3 represent in diagrammatic form the chemical processes which a molybdenum substrate body coated in accordance with the teachings of the prior art systems undergoes over a period of time.

FIGS. 4 and 5 represent in diagrammatic form the chemical processes which a molybdenum substrate body coated in accordance with the teaching of this invention undergoes over a period of time.

The objects of this invention are attained through the modification of the metal or metal alloy substrate body to which the coating of silicide is applied. This modification comprises adding to the substrate body material an amount of silicon which is equivalent to the silicon content of the equilibrium solution concentration of silicon in the metal or alloy for the temperature at which it is to be exposed during use. Since this amount of silicon forms a very small portion of the total weight of the refractory metal or alloy, it has little or no influence on the physical properties of the metal or alloy. However, its presence prevents the diffusion of the silicon from the coating into the metal or alloy substrate body and hence prevents the deterioration of the coating itself.

Turning to FIGS. l-3 it will be seen what mechanisms are involved in the deterioration of coatings applied to refractory metal or alloy substrates when the substrate is not modified. Assuming for example that the substrate body is pure molybdenum it will be seen that when a coating of molybdenum disilicide, MoSi is applied to it, there results a system which may be represented as in FIG. 1. Oxidation of this system for several hours at elevated temperatures gives rise to a system which is represented diagrammatically in FIG. 2. There is of course no attempt to show relative thicknesses in any of these figures. Under these conditions the silicon in direct contact with the oxidizing atmosphere reacts to form a glassy continuous film of SiO while that in the coating under the SiO later begins to diffuse into the molybdenum, forming a series of silicides and establishing a silicon gradient through the coating. With continued heating substantially all of this silicon diffuses into and throughout the molybdenum substrate thus in effect destroying any discrete layer of any one of the silicides of molybdenum to give the system represented in FIG. 3. Similar sys tems can be drawn for tungsten or for alloys containing molybdenum, tungsten or both of these refractory metals.

The equilibrium solution concentrations of silicon in molybdenum and in tungsten vary with temperature. In the case of molybdenum this concentration reaches a maximum at about 5 atomic percent of silicon in the terminal solid solution at about 2070 C. and in the case of tungsten at about 8 atomic percent of silicon at about 2250" C. These then represent the maximum amount of silicon which must be added to pure molybdenum and tungsten to modify them in accordance with this invention. It will be appreciated that both the molybdenum and/or tungsten content of an alloy Will determine the maximum solution concentration and that the temperature at which a refractory metal or alloy is to be used will determine the optimum amount of the silicon added. It is normally preferable not to add any more silicon than that which will just establish the required equilibrium conditions, for excess silicon may detract from the desired physical properties of the metal or alloy substrate. In general the preferred range for molybdenum is from about 15 atomic percent, for tungsten from about 4 to 8 atomic percent, and for alloys that amount which corresponds to the silicon solubility in the alloy at the temperatures employed.

With the addition of the specified amount of silicon to the metal or alloy, there results a system such as shown diagrammatically for pure molybdenum in FIG. 4, assuming that the silicide coating is in the form of Mo Si. The original coating of Mo Si becomes oxidized on the surface to form a film of SiO as in the case of FIG. 2, but no appreciable amount of the silicon diffuses to the substrate because of the equilibrium conditions existing at the interface. Hence, the protective layer of Mo Si remains intact even after prolonged heating. This is represented in FIG. 5.

In forming the modified substrate body of this invention any suitable technique may be used to incorporate the required amount of silicon into the substrate refractory metal or alloy. For example, a physical mixture of powdered silicon and molybdenum in the desired ratio was mixed and heated in a boron nitride mold at 1000 C. in an inert atmosphere for about one-half hour. The resulting sintered material had a density between about 60 and 85% of theoretical density. The sintered material was then zone refined according to well-known methods in an argon atmosphere. The zone refined modified molybdenum suitable as a substrate material for coating.

Suitable modified molybdenum can also be made by powdered metallurgical techniques such as that described in Metal Ind. (London) 75, 411,439 (1949), by diffusing the silicon into the metal or alloy which is accomplished by coating the substrate body with silicon and heating it, or by hot pressing a mixture of powdered silicon and molybdenum at a temperature of 1000 C. or higher.

The coating applied to the modified molybdenum body or substrate may be one or more of the molybdenum silicides, Mo Si, Mo Si or MoSi They may be app-lied by pack cementation, by vapor deposition, or by dipping in liquid silicon. In using the technique of pack cementation, for example, a mixture of powdered silicon and sodium fluoride is packed around the refractory metal or alloy body to be coated and the assembly is heated to about 1200 C. while hydrogen gas is passed around it. In this treatment silicon fluoride is first formed which then deposits a layer of silicon on the surface which in turn reacts with the molybdenum of the substrate body to form one or more of the molybdenum silicides. In

using the vapor deposition technique the substrate body is exposed to silicon fluoride and hydrogen vapor under a small pressure e.g., a few centimeters.

Prestabilization of the oxidation protection coating to form Mo Si may be preferable for many applications.

This can be done by heat treatment of the coated metal or alloy at a suitable temperature, near 1400 C., which will equilibrate coating and metal or alloy so that a layer of Mo Si is formed at least in immediate contact with the alloy. The equilibration step may be done in an inert atmosphere, in a vacuum, or in an oxidizing atmosphere. In an inert atmosphere, a higher temperature may be used for equilibration than in vacuum, since the atmosphere suppresses any loss of silicon by vaporization. In an oxidizing atmosphere preoxidation to form a protective layer of glassy silica can be accomplished simultaneously with formation of Mo Si.

Stabilization to Mo Si may be desirable for those uses where the coated substrate body is to be cycled through a temperature region where Mo Si and MoSi are subject to pest failurei.e., oxidative disintegration.

The silicide coating formed on the modified substrate body should preferably be no greater than about 10 mils thick. Coatings which are much thicker than this begin to influence the mechanical properties of the substrate body and normally this is undesirable. For practical purposes, the silicide coatings will usually be at least one mil thick.

It is also within the scope of this invention to construct a substrate body, e.g., gas turbine blade, having a core of any suitable material (molybdenum metal, tungsten metal or other high-temperature metal) on which there is a layer of the modified substrate material of this invention.

There are a number of molybdenumand tungstencontaining alloys which are used in high temperature applications. For example an alloy known as TZM (0.5% titanium, 0.08% zirconium and 99.4% molybdenum) is well-known and widely used. In modifying this refractory alloy for use as a substrate body in accordance with this invention, it is necessary to add from 1 to 5 atomic percent silicon based on the solubility of silicon in the alloy.

In like manner, if tungsten or a tungsten alloy is used as the substrate body then coatings of one or more of the tungsten silicides, WSi and W Si are built up as a coating by one of the techniques described for the molybdenum silicide coatings. These tungsten silicides would also be covered with a thin layer of SiO after oxidation.

The coated substrate body of this invention has the important inherent advantage of self-healing or selfregenerating. This is a result of the fact that there is always silicon available in the coating for reaction with oxygen to form SiO Where the originally formed SiO may have chipped off. This is possible since with this type of substrate one or more of the molybdenum silicides is always present at the coating-substrate interface. An examination of FIG. 3 shows that this is not the case for the prior art systems.

It will be seen from the above discussion that there is provided an improved refractory metal or alloy system which is protected against the action of heat and oxidizing atmospheres for an extended period and which at the same time retains the structural properties of the original refractory material. The system also provides for regeneration of the thin protective coating should this be necessary.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are eificiently attained and, since certain changes may be made in carrying out the above method and in the article set forth without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which as a matter of language might be said to fall therebetween.

We claim:

1. A refractory body suitable for use at elevated temperatures in oxidizing conditions, comprising in combination (a) a body substrate containing a refractory metal selected from the group consisting of molybdenum and tungsten and having dispersed therethrough a quantity of silicon in an amount substantially equivalent to the equilibrium solution concentration of silicon in said refractory metal at the temperature at which said body is to be used; and

(b) a coating on the surface of said body substrate of at least one of the silicides of said refractory metal.

2. A refractory body in accordance with claim 1 further characterized by having a thin layer of SiO;; covering substantially the entire surface of said coating.

3. A refractory body in accordance with claim 1 wherein said refractory metal is molybdenum and the amount of silicon contained in said body substrate ranges between about 1 and 5 atomic percent of said molybdenum.

4. A refractory body in accordance with claim 1 wherein said refractory metal is tungsten and the amount of silicon contained in said body substrate ranges between about 4 and 8 atomic percent of said tungsten.

5. A refractory body in accordance with claim 1 wherein said body substrate is a molybdenum alloy containing small amounts of titanium and zirconium.

6. A refractory body in accordance with claim 1 wherein said coating is no greater than 10 mils thick.

7. A refractory body in accordance with claim 3 wherein said coating contains Mo Si as its major constituent.

8. A refractory body in accordance with claim 3 further characterized by having a thin layer of SiO covering substantially the entire surface of said coating.

9. A refractory body suitable for use at elevated temperatures in oxidizing conditions, comprising in combination (a) a core;

(b) a substrate surrounding said core, said substrate containing a refractory metal selected from the group consisting of molybdenum and tungsten and having dispersed therethrough a quantity of silicon in an amount substantially equivalent the equilibrium solution concentration of silicon in said refractory metal at the temperature at which said body is to be used; and

(c) a coating on the surface of said body substrate of at least one of the silicides of said refractory metal.

10. A method of making a refractory body, comprising the steps of 6 (a) dispersing in a body substrate containing molybdenum a quantity of silicon in an amount between about 1 and 5 atomic percent of said molybdenum; and

(b) packing said substrate in a mixture of powdered silicon and a source of fluoride ions and heating the packed substrate while hydrogen gas is passed around it at elevated temperatures thereby to form on the surface of said substrate a thin layer of silicon which upon further heating reacts with said molybdenum in said body substrate to form at least one of the silicides of molybdenum as a surface coating for said body substrate.

11. A method in accordance with claim 10 including the step of exposing said coating to an oxidizing atmosphere thereby to form on it a substantially continuous thin layer of SiO References Cited UNITED STATES PATENTS 2,771,666 11/1956 Campbell et al. 117-106 X 2,855,328 10/1958 Long l17-l06 2,902,392 9/1959 Fitzer 117-71 X 3,015,579 1/1962 Commanday et al. 117-71 X 3,037,883 6/1962 Wachtell et a1 117-107.2 3,117,846 1/1964 Chao 117-71 X 3,192,065 6/1965 Page et al 1171 14 3,249,462 5/1966 Jung et a1 117135.1 X 3,269,856 8/1966 Jones 11770 3,307,964 3/1967 Jacobson 117l35.1 X

RALPH S. KENDALL, Primary Examiner. I. R. B'ATTEN, IR., Assistant Examiner. 

1. A REFRACTORY BODY SUITABLE FOR USE AT ELEVATED TEMPERATURES IN OXIDIZING CONDITIONS, COMPRISING IN COMBINATION (A) A BODY SUBSTRATE CONTAINING A REFRACTORY METAL SELECTED FROM THE GROUP CONSISTING OF MOLYBDENUM AND TUNGSTEN AND HAVING DISPERSED THERETHROUGH A QUANTITY OF SILICON IN AN AMOUNT SUBSTANTIALLY EQUIVALENT TO THE EQUILIBRIUM SOLUTION CONCENTRATION OF SILICON IN SAID REFRACTORY METAL AT THE TEMPERATURE AT WHICH SAID BODY IS TO BE USED; AND (B) A COATING ON THE SURFACE OF SAID BODY SUBSTRATE OF AT LEAST ONE OF THE SILICIDES OF SAID REFRACTORY METAL. 